LED illuminated waveguide projector display

ABSTRACT

There is provided a projection display ( 200 ), and a method for illuminating a projection display ( 200 ). The projection display ( 200 ) comprising a waveguide ( 2 ) comprising an input grating ( 4 ) having a plurality of linear diffractive features ( 6 ), the input grating ( 4 ) configured to couple in light into the waveguide ( 2 ), and an array of LEDs configured to form an illumination pupil which is optically relayed as an input pupil ( 8 ) onto the input grating ( 4 ), such that at the input grating ( 4 ) the input pupil ( 8 ) has a shape that is larger in a direction parallel to the linear diffractive features ( 6 ) than in a direction perpendicular to the linear diffractive features ( 6 ).

CLAIM OF PRIORITY

This application is a U.S. national-phase application filed under 35U.S.C. § 371 from International Application Serial No.PCT/GB2020/052600, filed on Oct. 15, 2020, and published as WO2021/094706 on May 20, 2021, which claims the benefit of priority toUnited Kingdom Application Serial No. 1916369.0, filed on Nov. 11, 2019,each of which are incorporated herein by reference in their entireties.

FIELD

The present invention relates to projector displays comprising anilluminator and a waveguide.

BACKGROUND

The use of diffractive waveguides for imaging displays is well known.Imaging displays of this type can be subject to a number of issues thatresult in undesirable effects arising in the output image. In addition,it is desirable to improve the efficiency of these systems.

FIG. 1 shows a prior art projection display 100 that utilises adiffractive waveguide 2. The waveguide 2 has an input grating 4 and anoutput grating 10 spaced apart from the input grating 4. The inputgrating has a plurality of linear diffractive features 6.

When in use, a projector (not shown) projects light onto the inputgrating 4 to form an input pupil 8. The input pupil 8 is circular inshape. The input grating 4 diffracts the light of the input pupil 8towards the output grating 10 through a series of total internalreflections within the waveguide 2. At the output grating 10 the inputpupil 8 is replicated as a series of output pupils 12. At eachinteraction of the light with the output grating 10 some of the light isdiffracted out of the waveguide as an output pupil replication 12forming the image. The rest of the light continues to form furtherdiffractions with the output grating 10 forming further output pupils12.

The size of the input grating 4 is typically optimised to maximise theefficiency of the in-coupled light. This ensures that the maximum amountof light is incident on the input grating 4 such that it is not lost.However, if the input pupil is too large the undesirable effect ofreinteraction may occur. This is where the diffracted light re-interactswith the input grating 4 after its initial diffraction by the inputgrating 4.

Reinteraction is shown in FIG. 2A, which shows a side on view of thewaveguide 2 of FIG. 1 . As can be seen, after its initial diffraction bythe input grating 4 the light experiences a single total internalreflection before again being incident on the face of the waveguide onwhich the input grating 4 is located. As the shortest ray walk 14 isshorter than the length 16 of the input grating 4 between interactionsthe light reinteracts with the input grating 4 as shown in point 15.This results in a loss of efficiency as some of this reinteracted lightis lost.

FIG. 2B shows a waveguide 2 where the shortest ray walk 14 is longerthan the distance between the first point of interaction with the inputgrating 4 and the end of the input grating 4. Thus, no interactionoccurs.

The length of the shortest ray walk 14 is dependent on the width of thewaveguide 18. Clearly the thicker the waveguide 2 the longer theshortest ray walk, as the light has to travel in the waveguide 2 furtherbefore it is reflected off the face of the waveguide. This means lesschance of reinteraction occurring for the same size input grating 4.However, for many applications it is desirable to have thin waveguides 2for efficiency purposes and/or for manufacturing requirements.

Simply reducing the size of the input pupil is one option for reducingre-interaction. As for a smaller pupil the length of the input gratingcan be reduced. However, if pupils are too small undesirable gaps in theoutput image can occur causing a “banding” effect. In addition,chromatic dispersion in the direction perpendicular to the lineardiffractive features always results in some broadening of the inputpupil in this dimension.

Thus, it is desirable to address the issue of reinteraction particularlyfor waveguides where it is not possible to reduce their thickness,whilst also ensuring banding is avoided.

SUMMARY OF INVENTION

According to an aspect of the invention, there is provided a projectiondisplay comprising: a waveguide comprising an input grating having aplurality of linear diffractive features, the input grating configuredto couple in light into the waveguide; and an array of LEDs configuredto form an illumination pupil which is optically relayed as an inputpupil onto the input grating, such that at the input grating the inputpupil has a shape that is larger in a direction parallel to the lineardiffractive features than in a direction perpendicular to the lineardiffractive features.

Forming an input pupil with a shape larger in a direction parallel tothe linear diffractive features of the input grating rather thanperpendicular to the linear diffractive features helps to avoidre-interaction. This is owing to the fact that a small input pupil inthe dimension perpendicular to the linear diffractive features reducesthe chance of reinteraction.

Reinteraction is not a problem in the dimension parallel to the lineardiffractive features. Thus, having an input pupil that is larger in thedirection parallel to the linear diffractive features enables theoverall size of the input pupil to be larger without experiencingreinteraction. It is desirable to have a large pupil, as if the pupil istoo small it can cause banding in the output image. Maximising the sizeof the input pupil in the direction parallel to the linear diffractivefeatures increases the overall size of the input pupil. This reducesbanding, but not at the expense of experiencing reinteraction.

The projection display may further comprise a tapered light pipe array,positioned between the array of LEDs and the remainder of the projectoroptics, wherein the tapered light pipe array is configured to receivelight from the array of LEDs before being optically relayed as the inputpupil on the input grating.

In this way, light emitted from each of the LEDs is efficientlycollected into a corresponding tapered light pipe. The light pipes actas a conduit to expand the light beams in two dimensions (2D) whilstreducing the angle of emittance of the light, i.e. the numericalaperture of the light is reduced. The light has a larger area but isemitting over a much smaller range of angles at the exit aperture at thesecond end of the tapered light pipes, than the light incident on thefirst end of the light pipe from the LEDs.

The reduction in angular range allows any subsequent lenses to relay animage of the exit aperture of the light pipe to the waveguide withgreater efficiency and precision than without a light pipe. Such anefficient optical arrangement is normally unacceptably large for compactprojectors.

Preferably, each LED of the array of LEDs has a respective tapered lightpipe.

As each LED has a corresponding tapered light pipe the array of taperedlight pipes are preferably arranged in the same pattern as thearrangement of the LED array. The tapering of the light pipes isachieved through having the first end, where light is received from theLED, smaller than the second end from which the light is emitted. Thetapered light pipe also expands the beam of light transmitted from theLED. In some cases this expansion may be an increase in width of thebeam of ×3, this corresponds to an ×9 expansion of the area of the lightincident on the first end of the tapered light pipe compared to thelight emitted from the second end of said light pipe. However, in otherembodiments the expansion may be any increase in beam width, forinstance ×2, ×4, ×5.

The end of each tapered light pipes may have a shape that is the same asthe shape of the input pupil. The shape of the input pupil at thewaveguide may be formed by the shape of the tapered light pipe. Thelight transmitted from the LEDs upon entering the tapered light pipe maynot have the desired shape. The tapered light pipe may ensure that thelight has a shape that is larger in a direction parallel to the lineardiffractive features than in a direction perpendicular to the lineardiffractive features of the input grating. For instance, the aspectratio of the end of the tapered light pipes is preferably 16:9.Alternatively, the aspect ratio could be any of 5:4, 4:3, and 16:10. Inaddition, the shape of both ends of each of the tapered light pipes mayalso be shaped to match the input pupil.

Alternatively, in other embodiments the shape of either end of the lightpipe may differ from the shape of the input pupil. In these embodimentsan optical component may be provided between the second end of the lightpipe (closest to the waveguide) and the waveguide to form the shape ofthe input pupil.

Alternatively, or in addition, the array of LEDs may be arranged on a 2Dsurface, wherein the shape of the spatial arrangement of the array ofLEDs is the same as the shape of the input pupil. For instance, thearray of LEDs may have any of the aspect ratios as discussed above. Thearray of LEDs may be arranged in an elliptical shape. Alternatively,they may be arranged in an elliptical shape. By arranging the array ofLEDs in an elliptical arrangement the illumination pupil from the arrayof LEDs will be of the shape of the input pupil. Alternatively, othermeans may be provided to create the shape of the input pupil.

Preferably, the waveguide has a width and the input grating has alength, and the input pupil's size is selected dependent on the width ofthe waveguide and the length of the input grating, such that the lightonly has a single interaction with the input grating. This ensures thatthe input pupil is small enough such that it does not causereinteraction with the input grating. Thus, the light does not interactwith the input grating once it has been diffracted and totallyinternally reflected. In this arrangement the width of the input gratingis defined as being perpendicular to the length of the input grating.The shortest ray walk is defined as the distance between the firstinteraction of the light with the input grating and the subsequentinteraction of the light with the waveguide after a single totalinternal reflection has occurred. The shortest ray walk is determined bythe width of the waveguide. A thicker waveguide provides a largershortest ray walk than a thinner waveguide. This is because in a thickerwaveguide the light has further to travel before interacting with thewaveguide. In many cases it may be desirable to make the width of thewaveguide thin or have a certain thickness. This may be due to designconstraints or to improve performance. However, a thinner waveguiderequires a smaller input pupil in the direction perpendicular to thelinear diffractive features of the input grating to avoidre-interaction.

The projection display may comprise a plurality of arrays of LEDs, eacharray of LEDs emitting light of a specific colour. In this way, inputpupils of specific colours can be created at the input grating. Thecolours of the LEDs may be red, green and blue. Alternatively, they maybe red, yellow and blue. Each array of LEDs may emit at a specificwavelength or range of wavelengths. Each array of LEDs may emit light ofa different colour to each of the other array of LEDs.

The array of LEDs may be of different colours distributed throughout thearray of LEDs. This removes the need for a dichroic combiner to combinelight of different colours. The colours of the LEDs may be red, greenand blue (RGB). In some embodiments, the different colours of LEDs maybe arranged in a repeating pattern. In other embodiments, the LEDs ofdifferent colours may be distributed randomly across the array. The LEDarray may contain the same number of LEDs of each colour. Alternatively,where the LEDs are micro LEDs there may be more green LEDs owing to thegreater efficiency of green micro LEDs.

Preferably, the waveguide is a plurality of waveguides, the inputgrating of each waveguide configured to couple in light of a differentcolour to the input gratings of each of the other waveguides. By havinga stack of waveguides each waveguide may be arranged to transmit lightof a specific colour. The input grating of each waveguide may bespecifically configured to couple in only a single colour of light.Alternatively, each waveguide may couple in multiple colours.

The plurality of arrays of LEDs may be arranged on a 2D surface, eacharray of LEDs offset from each other array of LEDs on said 2D surface.The surface may be a printed circuit board, or any type of surface knownto the skilled person.

The array of LEDs may be arranged on the 2D surface in a pattern of rowsof LEDs in a first axis, and columns of LEDs in a second axis. In thisway the LEDs are arranged across the 2D surface. This may be in anordered pattern. Alternatively, the LEDs may be arranged randomly acrossthe 2D surface.

Preferably, each row of LEDs in the LED array is offset with respect toits adjacent rows of LEDs. Each column may also be offset with respectto its adjacent columns. In other words, each row is out of phase fromits two nearest rows. In some embodiments this may be 90° out of phase.

In some embodiments, the array of LEDs may be arranged in a hexagonalpacking format. Advantageously, this hexagonal packing, or honeycombstructure, enables the closest packing of the LEDs in the smallestamount of space. This provides the largest density of LEDs in thesmallest area, maximising the useful LED area and so enhancing theefficiency of the projection display.

Preferably, the input grating of each waveguide is offset from the inputgratings of each of the other waveguides, such that only light of asingle colour is incident on each input grating.

The position of each of the input gratings may be offset by the sameamount as the corresponding array of LEDs of that colour. In this way,each of the input gratings arranged to receive a particular colour oflight is offset such that it is aligned with the array of LEDs of thatcolour. This means that light of a single colour is incident on eachinput grating. This improves efficiency as light is not lost throughunwanted interactions with input gratings that are not configured tocouple in that colour of light. The input gratings may be offset in adirection perpendicular to the width of the waveguide. Alternatively, orin addition, they may be offset in a direction parallel to the width ofthe waveguide.

Preferably, the plurality of waveguides comprise a first waveguide, asecond waveguide and a third waveguide, the input grating of the firstwaveguide configured to couple in red light, the input grating of thesecond waveguide configured to couple in blue light, and the inputgrating of the third waveguide configured to couple in green light.Alternatively, the plurality of waveguides may be two waveguides.Alternatively, they may be configured to couple in other colours. Forinstance, red, yellow and blue.

The shape of the input pupil may be elliptical or rectangular. An inputpupil having this shape avoids reinteraction whilst reducing banding. Italso provides improved overlaps of the replications of the input pupilin the output grating.

Preferably, the waveguide further comprises an output grating, theoutput grating configured to receive light from the input grating andreplicate the input pupil multiple times, to form an exit pupil couplingthe light out of the waveguide.

The array of LEDs may be an array of microLEDs. By using micro LEDscoupled with a microlens array or array of micro tapered pipes, the sizeof the collection optics can be reduced. The efficiency of the device isalso improved compared to other types of light sources or conventionalLEDs. LEDs are Lambertian emitters i.e. they emit light over a widerange of angles, typically 2π steradians. MicroLEDs, when coupled with amicrolens or micro-taper array are not Lambertian emitters and emit overa much smaller range of angles, resulting in less light lost. The devicecan provide double to triple the lumens per watt compared to lightengines using currently known compact illuminator designs.

Preferably the linear diffractive features are grooves. Alternatively,they may be volume holograms or any other type of linear diffractivegrating.

Preferably, the projection display further comprises a liquid crystal onsilicon, LCOS, display positioned between the array of LEDs and thewaveguide. The LCOS display may be positioned in the light path betweenthe array of LEDs and the input grating of the waveguide. The LCOSdisplay may form an image with the light incident on it from the LEDs.The image may then be projected as the input pupil onto the inputgrating of the waveguide. Alternatively, any other type of imagegenerating means may be used. The display may be any type ofnon-self-emissive display, including transmissive liquid crystaldisplay, reflective liquid crystal display, digital light processingdisplays, or dynamic mirror arrays.

The illuminator pupil may be formed on the projector optics that relayan image of the illumination pupil on or near to the input grating. Thisrelayed image may form the input pupil.

According to a further aspect there is provided an augmented reality(AR), or virtual reality (VR) device, comprising the projection displayof the above aspect. Preferably, the AR or VR device is an AR or VRheadset.

According to a further aspect there is provided a method of illuminatinga projection display comprising: emitting light from an array of LEDs toform an illumination pupil; optically relaying the illumination pupil asan input pupil onto an input grating of a waveguide to couple the lightinto the waveguide, such that at the input grating the input pupil has ashape that is larger in a direction parallel to the linear diffractivefeatures of the waveguide than in a direction perpendicular to thelinear diffractive features of the waveguide; and projecting the lightout of the waveguide to form an image.

DESCRIPTION OF FIGURES

FIG. 1 is schematic face-on view of a prior art waveguide projectiondisplay;

FIGS. 2A and 2B are schematic side-on cross-section views of a prior artwaveguide projection display;

FIG. 3A is a schematic face-on view of a waveguide projection displayaccording to an embodiment of the invention;

FIG. 3B is schematic face-on view of the waveguide of the projectiondisplay as shown in FIG. 3B.

FIG. 4 is a schematic side-on view of a further example projectiondisplay according to an embodiment of the invention;

FIG. 5 is a schematic view of an example LED array of the projectiondisplay according to an embodiment of the invention;

FIG. 6 is a perspective view of a tapered light pipe for use in theprojection display according to an embodiment of the invention;

FIG. 7 is a schematic view of an example illuminator of the projectiondisplay in an embodiment of the invention;

FIG. 8A is a schematic side-on view of the projection display accordingto a further example embodiment of the invention; and

FIG. 8B is a schematic face-on view of the waveguide of the projectiondisplay as shown in FIG. 8A.

DETAILED DESCRIPTION

FIG. 3A is a schematic face-on view of a waveguide projection display200 according to an embodiment of the invention. The waveguideprojection display 200 includes a waveguide 2 and an illuminator (notshown due to the orientation of the Figure). The illuminator is in theform of an array of LEDs.

The waveguide 2 has an input grating 4 and an output grating 10. Theoutput grating 10 is spaced apart from the input grating 4. The inputgrating 4 has a plurality of linear diffractive features 6. The lineardiffractive features 6 are for diffracting the light incident on inputgrating 4. The linear diffractive features extend in a first directionthat is perpendicular to the path that the light takes once it hasdiffracted off the input grating. This is known as the across trackdirection. The along track direction is defined as perpendicular to thelinear diffractive features.

Light generated from the illuminator (not shown) creates an input pupil8 at the input grating. The illuminator's light source is an array ofLEDs. As can be seen the input pupil 8 has a shape that is larger in thedirection parallel to the linear diffractive features 6 of the inputgrating 4 (indicated by reference a), than in a direction perpendicularto the linear diffractive features 6 of the input grating 4 (indicatedby reference b). This is longer in the across track than along trackdirection. In the example shown in FIG. 3A the input pupil 8 has arectangular shape. However, any shape that is longer parallel to thelinear diffractive features than perpendicular to the linear diffractivefeatures could be used. Conventional input pupils are circular as shownin FIG. 1 . Reinteraction of the light with the input pupil is avoidedby keeping the size of the input pupil 8 small in the directionperpendicular to the linear diffractive features. However, having alarge input pupil 8 in the direction parallel to the linear diffractivefeatures 6 ensures that the overall size of the input pupil 8 is not toosmall, avoiding the issues experienced by having a banded output throughhaving too small a pupil.

After the light has been diffracted by the input grating 8, it isreflected towards the output grating 10 through a series of totalinternal reflections. Once at the output grating 10 the light diffractscausing replications of the input pupil as output pupil 12 each couplinglight out of the waveguide. At each interaction with the output grating10 some of the light continues to create further replications of theinput pupil 12, resulting in the desired image expansion. The path ofthe light is represented by the lines in FIG. 3A shown on top of theoutput pupils (as in FIG. 1 for the prior art system).

As can be seen from FIG. 3A the output pupils 12 at the output grating10 have substantially the same shape as the input pupil 8 at the inputgrating 4. Advantageously, the rectangular shape of the output pupils 12shown in FIG. 3A result in a much better overlap of the pupilreplications at the output grating 10, than the circular pupilreplications of the prior art as shown in FIG. 1 . This forms a smootherand more uniform image without banding.

FIG. 3B shows a side-on view of the projection display 200 of FIG. 3A.Waveguide 2 has input grating 4 and output grating 10 as describedabove. LED array 20 is shown arranged on 2D surface 22. The LED array 20generates light which is incident on input grating 4 forming the inputpupil 8 as described above.

FIG. 4 shows a further example of how the waveguides may be arranged inan example projection display 300 according to the present invention. Inthis example there are three waveguides 2 a 2 b and 2 c, each having arespective input grating 4 a 4 b 4 c and output grating 10 a 10 b 10 c.In the example shown in FIG. 4 each waveguide is configured to transmita different colour to each of the other waveguides. Waveguide 2 a andits gratings 4 a and 10 a are configured to transmit blue light,waveguide 2 b and its gratings 4 b and 10 b are configured to transmitgreen light, and waveguide 2 c and its gratings 4 c and 10 c areconfigured to transmit red light. In this arrangement, LED array 20transmits light in red, green and blue.

The LED array 20 is configured to transmit light onto the input gratingof each of the waveguides 2 a 2 b and 2 c to form an input pupil 8having the shape as described above in relation to FIG. 3A.

FIG. 5 shows an example layout of the array of LEDs 20 shown in thearrangements in FIGS. 3B and 4 .

The array of LEDs 20 is shown arranged on surface 22. FIG. 5 just showsan example portion of the surface 22 and, the surface 22 may containhundreds of LEDs in a repeating pattern. This rectangular arrangement ofthe array of LEDs forms the input pupil with its desired rectangularshape.

It can be seen that the LED array 20 is arranged in rows 48 a 48 b, 48 cand 48 d, and columns 40 a, 40 b, 40 c. The LEDs in row 48 b are offsetfrom the LEDs in rows 48 a and 48 c. Likewise each column 42 b is alsooffset with respect to adjacent columns 42 a 42 c. This regularrepeating pattern would expand across the surface 22 such that the LEDsare closely packed.

The distance between each LED, otherwise known as the pitch, is shown byarrow 44. The pitch is larger than the size of each individual LEDindicated by arrow 46. This space between LEDs may be provided toaccommodate optics that couples the light from the LEDs, such as lightpipes.

The letters “R” “B” and “G” on the LEDs 42 a 42 b 42 c indicate thecolour of the LEDs 42 a 42 b 42 c. The LEDs of different colours aredistributed across the LED array 20.

As discussed above, the shape of the input pupil 8 formed at the inputgrating 4 of the waveguide 2 is larger in the direction parallel to thelinear diffractive features 6 of the input grating 4 than in thedirection perpendicular to the linear diffractive features 6. The inputpupil is the relayed image of the illumination pupil. The LED array canbe arranged spatially to match the input grating dimensions. Inaddition, anamorphic optics can also be used to change the proportionsof the LED array to match to the shape desired at the input grating.

An alternative or additional way to ensure that the input pupil 8 hasthe desired shape is through the use of light pipes. An example lightpipe is shown in FIG. 6 . As can be seen the tapered light pipe 30comprise a first end 34 and a second end 32, the light pipe 30 extendingalong its length L between the first 34 and second ends 32. As can beseen, the area of the first end 34 is smaller than the area of thesecond end 32, the length L of the light pipe 30 tapering between thetwo ends. The first end 34 of the light pipe 30 covers the output of anLED such that the greatest proportion of the light emitted from the LEDenters the light pipe 30.

As can be seen, the cross section of the first and second ends 32 34 aresuch that when the light exits the light pipe 30 it has a profile thatmatches the cross sectional shape of the second end 32 of the lightpipe. In this case this is rectangular. This means that the shape of thebeam of light exiting the light pipe 30 can form an input pupil 8 whenincident on the input grating that is larger in the direction parallelto the linear diffractive features of the input grating than in thedirection perpendicular to the linear diffractive features 6.

FIG. 7 shows an example illuminator portion 60 of the projection displayof the present invention. The illuminator portion 60 includes an LEDarray 20, a tapered light pipe array 30, a collimating microlens array50, a field microlens array 52, and a relay lens 54.

The LED array is arranged on printed circuit board surface 22; havingeach of the LEDs 42 a 42 b and 42 c arranged on said surface 22.

FIG. 7 also shows LCOS panel 56 which the illuminator 30 is used toilluminate. It can be understood that any type of non-emissive panelcould be used. The light from the LCOS panel is then reflected onto theinput grating 4 of the waveguide 2, as described above.

Each of the LEDs 42 a 42 b 42 c is coupled to a respective tapered lightpipe 30 a 30 b 30 c and a respective field 50 a 50 b 50 c andcollimating 52 a 52 b 52 c microlens.

The LED array 20 comprises red, green and blue LEDs. For simplicity onlya single LED of each colour LED is shown in FIG. 7 . However, the LEDarray 20 includes many LEDs of each colour. The LED array 20 may includeseveral hundred LEDs arranged on panel 22. For instance, the LED array20 may have 400 LEDs.

Each tapered light pipe 30 a 30 b 30 c is arranged in close proximity toits corresponding LED 42 a 42 b 42 c such that the light emitted by theLEDs 42 a 42 b 42 c is collected into the light pipe 30. Typically over90% of the light emitted from each LED 42 a 42 b 42 c is collected byits respective light pipe 30 a 30 b 30 c.

Each light pipe 30 a 30 b 30 c is tapered such that the end closest tothe LEDs 42 a 42 b 42 c is smaller than the end furthest from the LEDs42 a 42 b 42 c, as will be discussed below. Each of the tapered lightpipes 30 a 30 b 30 c is a solid pipe of transparent material.

Looking at LED 42 a and its corresponding light pipe 30 a, once receivedin the light pipe 30 a, the light emitted from the LED 42 a is totallyinternally reflected by the light pipe 30 a as it travels through thelight pipe down its length. Each time the light is totally internallyreflected, the angle of the light is decreased. Due to the taperednature of the light pipe 30 a, once the light exits the light pipe 30 a,the beam of the light has enlarged, whilst the angle of the beam hasreduced.

The light that exits the light pipe 30 a is then incident on a fieldmicrolens 50 a, which improves the uniformity of the beam, imaging theexit pupil of the light pipe 30 a. The light is then incident on thecollimating microlens 52 a which collimates the light to generate acollimated image of the of the exit aperture of the light pipe 30 a.This light is then incident on relay lens 54 which projects the lightonto the LCOS panel 56, generating and focusing the collimated image ofthe exit aperture of the light pipe 30 a onto the panel 56.

Light is projected from each LED 42 a 42 b 42 c through itscorresponding light pipe 30 a 30 b 30 c, field microlens 50 a 50 b 50 c,and collimating microlens 52 a 52 b 52 c, as discussed above. Forinstance, light from LED 42 b passes through light pipe 30 b, fieldmicrolens 50 b, collimating microlens 52 b, then onto relay lens 54, andlight from LED 42 c passes through light pipe 30 c, field microlens 50c, collimating microlens 52 c, then onto relay 54. The single relay lens54 projects this light across the LCOS panel 56. Thus, an image of eachof the emitting ends (exit apertures) of the light pipes 30 a 30 b 30 cis projected across the entire LCOS panel 56 display, resulting in asuperposition of said images, covering the entire area of the panel 56.

This can be seen in FIG. 7 where the light from LED 42 a and light pipe30 a is projected onto the panel at each of points 58 a 58 b and 58 c.Similarly, the light from LED 42 b and light pipe 30 b is projected ontothe panel at each of points 58 a 58 b and 58 c, as is the light from LED42 c and light pipe 30 c. Thus, the panel 56 is homogeneouslyilluminated by the array of LEDs 20 and tapered light pipes 30, thesuperposition of each of the light paths providing the desired intensityat the panel 56. Although FIG. 7 shows the light from each LED beingincident on discrete portions of the image (i.e. points 58 a 58 b 58 c),in practice the distance between such points is small enough such thatthe entire display appears illuminated.

Having an array of LEDs, corresponding light pipes and microlensesallows the volume of the illumination optics to be reduced, allowing anefficient illuminator design in a far smaller volume. This enables ahomogenised uniform irradiance on the display with both high efficiencyand low volume.

As outlined above, the LEDs may be microLEDs. Miniaturising thisefficient design using micro LEDs means that the length of theilluminator is reduced. However, the overall volume scales linearitywith miniaturisation factor. Thus, the illuminator is more compact thanif a single light source was to be used. As each light pipe in the lightpipe array is imaged across the whole of the display, having an arrayensures that the entrance pupil of the projector is filled andsufficient intensity of illumination is achieved. Each typical micro LEDmay have an area of less than 0.04 mm².

The colours (R, G, B) shown in FIG. 7 are purely for illustrativepurposes and the LEDs 42 a, 42 b and 42 c may be any colour. Forinstance, they may all be the same colour. For colour sequentialilluminators the light from each LED of each colour is emitted at adifferent point in time. In this colour sequential mode, the light fromthe LEDs 20 having the same colour will be incident on the panel 56together in the way as described above.

FIGS. 8A and 8B shows an alternative projection display 400 according toa further example of the present invention. The projection display 400has three waveguides 2 a 2 b and 2 c, each having a respective inputgrating 4 a 4 b 4 c. Waveguide 4 a is configured to propagate bluelight, waveguide 4 b is configured to propagate green light, andwaveguide 4 c is configured to propagate red light.

Projection display 400 also includes three LED arrays. Blue LED array 20a, green LED array 20 b, and red LED array 20 c. Each of the LED arraysare offset from each other on the panel 22. The offset is in the alongtrack direction, i.e. in the direction that the light propagates in thewaveguide between the input grating and the output grating. Each of thearrays of LEDs produces an input pupil at its respective input gratinghaving the shape as described above.

By having this staggered array of LEDs and input gratings splitting ofthe pupil by colour can be achieved. This arrangement of input gratingsmeans that light of a specific colour is only incident on the gratingthat is configured to diffract that colour of light. This increases theefficiency of the projector display, as light is not lost by unwanteddiffractions with input gratings not configured to diffract that colourof light. For instance, if the input gratings were aligned with oneanother then the red light would have to pass through input grating 4 band input grating 4 a, which are configured to diffract the green andblue light, respectively. Unwanted diffraction of the red light couldoccur in either of these gratings 4 a and 4 b, thus meaning that some ofthe red light is lost, reducing the efficiency of the system. Acorresponding situation could also occur for the green light withrespect to grating 4 a. By offsetting the input gratings this potentialissue is avoided. This may be through offsetting each of the array ofLEDs of a specific colour by the same amount as the corresponding inputgratings. By each input grating only receiving light of a single colourthe input grating need not be transparent to the other colours.

Although the input gratings shown in FIG. 8A (and FIGS. 3B and 4 ) areshown on the face of the waveguide nearest the array of LEDs, it wouldbe understood that they could alternatively be located on the oppositeface of the waveguide. In this alternative arrangement the light has topass through the waveguide before being diffracted by the input grating.This would allow the input gratings to be metalized. Metalized inputgratings do not allow light to pass through the grating, they only allowdiffraction of the light. This can further increase the couplingfrequency of the input gratings.

In other arrangements, rather than the array of LEDs being offset by thesame amount as the offset of the input gratings, additional optics maybe provided to ensure that each waveguide only receives light of asingle colour.

FIGS. 3A, 4 and 8A show the projector system of the present inventionwith an array of LEDs transmitting light directly onto the input gratingof the waveguide. It will be understood that this may be a simplifiedarrangement and other optics may be between the LEDs and the waveguide.One such arrangement may be the arrangement shown in FIG. 7 . However,other arrangements may be used. To generate an image at the output ofthe waveguide a device for generating an image from the LED light may berequired. This may be a LCOS or LCD type projector. The output of thisdevice relays the light from the LED array to form the input pupilhaving the required shape at the input grating as described above.

By having an array of LEDs 20 and an array of light pipes 30 the lengthL of each light pipe can be small. This is further enabled by using anarray of LEDs where each LED is of a small size, for instance, microLEDs, as smaller LEDs emit over a smaller area. The size of the LED isdirectly proportional to the length of the optical path required (i.e.the distance between the LED and the panel 56). Thus, by using a microLEDs, each of which has a 1/20 the size of a traditional light source,this enables the optical path to be reduced by 1/20. This provides anenhanced saving in volume of up to 95%. For instance, an optical paththat is traditionally 40 mm long can be reduced to 2 mm.

Due to the reduction in size of the illuminator, the projection displaycan be a handheld or wearable device. For instance, the projectiondisplay may be a virtual reality or augmented reality headset, wheresize constraints are of paramount importance.

Having described aspects of the disclosure in detail, it will beapparent that modifications and variations are possible withoutdeparting from the scope of aspects of the disclosure as defined in theappended claims. As various changes could be made in the aboveconstructions, products, and methods without departing from the scope ofaspects of the disclosure, it is intended that all matter contained inthe above description and shown in the accompanying drawings shall beinterpreted as illustrative and not in a limiting sense.

In the above the term colour has been used when referring to the lightemitted by the LEDs and transmitted by each waveguide. This refers tothe wavelength of the light. There is typically a range of wavelengthsassociated with a colour.

Alternative arrangements may be used to create the desired shape ofinput pupil, not just the shape of the light pipe. For instance, a rangeof optics, such as lenses may be used to ensure that the light has thedesired shape. Alternatively, the LEDs themselves may have the desiredshape such that they emit with the desired shape without the need foradditional optics.

Although the light pipe 30 shown is a solid pipe of transparentmaterial, with the beam of light totally internally reflected along theinternal length of said pipe, any type of light pipe that causes thebeam of light to expand in 2D, whilst reducing the beam angle, may beused. For instance, the tapered light pipe 30 may be made from a hollowpipe with reflective internal sides.

The skilled person would understand that any lens array that providesthe function of the relay lens could be used, not just a single lens.For instance, this may involve having a plurality of relay lenses. Inaddition, although a field microlens array is shown in FIG. 7 thepresence of a field microlens array is not essential.

The shape of the end of the light pipes is not limited to beingrectangular as shown in the Figures. For instance, the end of the lightpipe may be elliptical.

The invention claimed is:
 1. A projection display comprising: awaveguide comprising an input grating having a plurality of lineardiffractive features, the input grating configured to couple in lightinto the waveguide; and an array of LEDs configured to form anillumination pupil which is optically relayed as an input pupil onto theinput grating, at the input grating the input pupil having a shape thatis larger in a direction parallel to the linear diffractive featuresthan in a direction perpendicular to the linear diffractive features. 2.The projection display of claim 1, further comprising: a tapered lightpipe array, positioned between the array of LEDs and the waveguide,wherein the tapered light pipe array is configured to receive light fromthe array of LEDs before being optically relayed as the input pupil onthe input grating.
 3. The projection display of claim 2, wherein eachLED of the array of LEDs has a respective tapered light pipe.
 4. Theprojection display of claim 3, wherein an end of each tapered light pipehas a shape that is the same as the shape of the input pupil.
 5. Theprojection display of claim 1, wherein the waveguide has a width and theinput grating has a length, and the input pupil's size is selecteddependent on the width of the waveguide and the length of the inputgrating, such that the light only has a single interaction with theinput grating.
 6. The projection display of claim 1, further comprisinga plurality of arrays of LEDs, each array of LEDs emitting light of aspecific color.
 7. The projection display of claim 6, wherein thewaveguide is a plurality of waveguides, the input grating of eachwaveguide configured to couple in light of a different color to theinput gratings of each of the other waveguides.
 8. The projectiondisplay of claim 7, wherein the plurality of arrays of LEDs are arrangedon a 2D surface, each array of LEDs is offset from each other array ofLEDs on said 2D surface.
 9. The projection display of claim 8, whereinthe input grating of each waveguide is offset from the input gratings ofeach of the other waveguides, such that only light of a single color isincident on each input grating.
 10. The projection display of claim 1,wherein the array of LEDs is arranged on a 2D surface, wherein the shapeof the spatial arrangement of the array of LEDs is the same as the shapeof the input pupil.
 11. The projection display of claim 1, wherein theshape of the input pupil is elliptical or rectangular.
 12. Theprojection display of claim 1, wherein the waveguide further comprisesan output grating, the output grating configured to receive light fromthe input grating and replicate the input pupil multiple times, to forman exit pupil coupling the light out of the waveguide.
 13. Theprojection display of claim 1, wherein the array of LEDs are an array ofmicroLEDs.
 14. The projection display of claim 1 further comprising anaugmented reality, or virtual reality device.
 15. A method ofilluminating a projection display comprising: emitting light from anarray of LEDs to form an illumination pupil; optically relaying theillumination pupil as an input pupil onto an input grating of awaveguide to couple the light into the waveguide, such that at the inputgrating the input pupil has a shape that is larger in a directionparallel to the linear diffractive features of the waveguide than in adirection perpendicular to the linear diffractive features of thewaveguide; and projecting the light out of the waveguide to form animage.
 16. The method of claim 15, wherein the shape of the input pupilis elliptical or rectangular.
 17. The method of claim 15, whereinprojecting light out of the waveguide further comprises: receiving lightfrom the input grating at an output grating; and replicating, throughthe output grating, the input pupil multiple times to form an exit pupilcoupling the light out of the waveguide.
 18. The method of claim 15further comprising receiving light from the array of LEDs at a taperedlight pipe array before optically relaying the illumination pupil as theinput pupil.
 19. The method of claim 18 further comprising shaping abeam of light exiting the tapered light pipe to have the shape of theinput pupil at the input.
 20. The method of claim 15, wherein emittinglight from an array of LEDs further comprises emitting light ofdifferent colors from the array of LEDs.